(tree) Add changeset for perf improvement to composition (#23929)

Author: yann-achard-MS

.changeset/blue-plants-act.md

@@ -0,0 +1,14 @@
+---
+"@fluidframework/tree": minor
+"fluid-framework": minor
+---
+---
+"section": tree
+---
+
+Faster creation of large transactions and faster processing of inbound changes.
+
+SharedTree sometimes composes several sequential changes into a single change.
+It does so whenever a transaction is created and when processing inbound changes.
+[This PR](https://github.com/microsoft/FluidFramework/pull/23902) makes this composition process asymptotically faster.
+For example, creating a transaction that performs 1000 edits on a single array now takes 170ms instead of 1.5s (an 89% improvement).